In the course of a normal day, the knee joint of an active individual is the one anatomical structure most stressed, and therefore, most prone to injury. While physical examination can determine the extent of most injuries, at the present time only a surgical arthroscopic examination can diagnose internal knee derangement and provide the basis for a well informed decision to perform surgey to repair the structure. However, an arthroscopic examination of the knee costs approximately $2,000 and does entail some risk of harm. Therefore, there is a need to find a less costly and equally effective technique of diagnosing injuries to the internal structure of the knee.
A knee joint is composed of both internal and external components. The internal components include the articular cartilages, or menisci, and the cruciate ligaments (both anterior and osterior.) The external components include the capsule and collateral ligaments. The knee joint operates to provide a force transmitting articulation between the upper leg bone (the femur), and the lower leg bone (the tibia). The knee cap, or patella, rests generally on the femur and is enclosed by the patellar tendon.
The integrity of the knee joint is maintained by the ligaments. The cruciate and the collateral ligaments are particularly important in this function. Of these ligaments, the cruciate ligaments are considered by far the most complex structure in the knee joint. The anterior and posterior cruciate ligaments cross each other at the center of the knee joint, and each is attached to both the femur and the tibia.
Any anterior or posterior movement of the proximal (or upper) end of the tibia relative to the distal (or lower) end of the femur is resisted by the tensile forces developed in the cruciates. Thus, forward (or anterior) movement of the tibia relative to the femur will result in increased tension in the anterior cruciate ligament.
Classically, injury to the anterior cruciate ligament (ACL) is caused by a deceleration twisting injury of the knee. This is associated with a "pop" from deep within the knee, an inability to continue any activity using the knee, and the development within 24-hours of a tense infusion (swelling) of the knee. If the syndrome is properly identified, diagnosis of ACL injury will be correct 90% of the time, with repair possible in 77% of the cases. For repair to be possible, it must be carried out within ten (10) days of the injury. However, these figures do not reflect how many cases of ACL injury go undiagnosed, leading to chronic knee instability in many cases.
Experiments conducted by the Department of Orthopedic Surgery at Kaiser Hospital and the University of California, both in San Diego, have demonstrated that the forward displacement of the tibia relative to the femur upon application of a force to the calf of the lower leg results in a displacement of the tibia relative to the femur in the anterior/posterior plane. The amount of displacement for a given force varies widely in the normal population. However, if it has been found that in the normal population there is less than a 2 millimeter displacement difference between displacement measured on the legs of a given individual for a given force, usually 20 pounds. If the difference in displacement betwen the legs of the individual is in the range of 2.0 to 2.5 millimeters, ACL disruption in the leg showing the largest displacement is suggested. If a 3 millimeter displcement difference exists between the legs of the individual, a diagnoses of ACL disruption is indicated. A summary of the studies is found in a paper prepared for the annual meeting of the American Association of Orthopedic Surgeons presented in Atlanta, Georgia in 1984.
Several techniques have been used in the past for applying forces to the legs of an individual to diagnose ACL injury manually. Neither of these manual tests are sensitive enough to accurately and reproducibly measure the difference in displacement between the legs of an individual. The anterior drawer test is performed with the knee flexed to 90.degree. and the foot stabilized. The examiner puts both hands around the upper end of the lower leg and draws the tibia toward himself to determine displacement. However, muxcular spasms of tense joint effusion can occasionally cause a false-negative anterior drawer sign. Therefore, the so-called Lachman's test, performed in a similar manner to the drawer test, but at 20.degree. of flexion is considered a more sensitive test.
At least one device has been suggested for performing the Lachman test with sufficient accuracy to measure the difference in displacement between the legs of an individual. This device is identified as a knee laxity tester and is manufactured by the Stryker Corporation of 420 Alcottstreet, Kalamazoo, Mich. However, a fully satisfactory device for quantitatively and reproducibly measuring lasity of the anterior cruciate ligament of the knee has not yet been developed. The device should combine safety for the individual tested with ahigh accuracy and reproducibility of test results.